A deep insight into petrichor, the smell that follows the rain


It's that familiar smell that comes after a light rain: petrichor. And as it turns out, there's a good reason we can smell it.

The term “petrichor” originally comes from a 1964 Nature magazine article in which a group of Australian researchers coined the word.

“Some Australian scientists coined the word petrichor,” said evolutionary biologist Dan Riskin. “Petra means stone and ichor means something like perfume. So it's the scent of stones, and it turns out it's not the stones themselves that give off that smell. But it's a pretty good name.”

Riskin says the smell actually comes from the soil. More specifically, from all the microorganisms in the soil.

“There are microbes, there are algae, there are fungi,” Riskin told CTV News. “And they all produce these little chemicals that are sprayed into the air like perfume, so when you sniff it, you don't smell the rocks or the rain. You smell the life that lives in those soils.”

Compared to other animals, humans do not normally have a strong sense of smell, but surprisingly they are very sensitive to petrichor, which is about 10 parts per trillion.

“The various articles I found said it was even more sensitive than sharks to a drop of blood in the ocean,” said Dr. Julia Boughner, a professor in the Department of Anatomy, Physiology and Pharmacology at the University of Saskatchewan. “That suggests this chemical is really important to humans.”

While humans perceive the smell and generally find it pleasant, according to Boughner, not all animals find it appealing.

“There is evidence that bees are attracted to the scent, but only in light rain, not heavy rain,” Boughner said. “And it seems that you can smell it better in light rain because it activates the chemicals. Fruit flies don't like it, mosquitoes do, and a tiny worm that we often use in science doesn't like it.”

One conclusion as to why humans like the smell of petrichor, according to Riskin, could be related to evolution and the ability to find soil in which things grow.

“If you think back to our ancestors who lived off the land and needed to know when the fruit would ripen or when the planted seeds would grow, that smell was of enormous importance,” he said.

In addition to its potential adaptation to lush landscapes, Boughner says petrichor has noticeable effects on humans.

“Petrichor has interesting effects because it has a relaxing effect, similar to forest bathing,” she said. “These chemicals actually produce a feeling of health and can be helpful for people in some cases. So just being in nature seems to be beneficial for us.”

Boughner finds it interesting that so many different animal species throughout history – even back to the Cambrian period with trilobites – have had the ability to respond to petrichor.

“To me, the idea that a tyrannosaur might have had receptors for petrichor and that other animals in the past lived in a way that detected environmental stimuli that we might be able to smell today is just incredible,” she said.

Riskin says it's nice to know the science behind beautiful things, and knowing the deeper purpose of detecting petrichor is a great example of that.

“You know a flower smells good and looks good, but then you understand that it actually evolved to attract animals and that that smell has a purpose,” Riskin said.

“It deepens the beauty. And I think petrichor is just a beautiful smell that everyone can enjoy. But when you think about the fact that our bodies sense when the land is ready to accept seeds, to grow things for us, it just adds a lot to the whole story and makes it even more beautiful.”